Location of the FUT2 gene, which codes for secretor status.
Secretion of ABH antigens is under control of two linked genes on gene locus 19q13, another Haplotype: a mutation (SNP variant) in one always goes with a mutation in the other. Presence of the secretor gene adds the H antigen (fucose) to red blood cells and body secretions. If this genetic code for secreting H is absent in the genetic material inherited from both parents, the individual will not secrete their red blood cell antigens into their body tissues, which is what we know as ABH non-secretors. If they inherit the secretor gene from one parent only, they may still have some of the characteristics of a non-secretor: even though they secrete their ABH antigens, it was postulated that they may have some of the metabolic disease associations connected with being a non-secretor (but not necessarily the cell surface antigen-related ones). 19q13 has 288 verified genes related to this locus, even more than the ABO locus. Chromosome 19 has the highest gene density of all human chromosomes, and many of these relate to how the immune system works, which explains the difference between immune response of secretors and non-secretors: the humoral vs. the cellular response (TH1 and TH2). Other potentially genes on this chromosome relate to insulin-dependent diabetes, familial hypercholesterolaemia, and repair of other genes relating to repairing DNA damage from exposure to radiation and to other environmental pollutants.
The probability of having a particular combination of two specific alleles at a given locus can be calculated using a mathematical formula, which suggests that 2/3 of the general population will be heterozygous for secretor status (i.e. having both secretor and non-secretor genes), which may have a significance in itself when compared with homozygote secretors and non-secretors.